Uncharted archives – imprints of tsunami backwash deposits on the Algarve shelf (Portugal)

<p>Research on offshore tsunami deposits is scarce and their depositional processes and preservation potential are virtually unexplored. Therefore, the RV Meteor cruise M152 mapped and sampled one coast-parallel and two coast-perpendicular transects at water depths from 65 to 114 m off the Algarve coast (Portugal). This coast was strongly affected by the well-known Lisbon earthquake and tsunami of November 1<sup>st</sup>, 1755 AD. Numerous onshore locations have been well documented and studied with historic damage reports and modern scientific investigations of the onshore tsunami deposits. However, very scarce information about the backwash, the water masses flowing back into the sea, exists and their imprint on the shelf is unexplored.</p><p>In order to fill this gap, a total of 19 vibracores were recovered during the RV Meteor cruise M152. For tracing the sedimentary imprint of the AD 1755 tsunami and potential predecessors, a multi-proxy analysis was carried out (sedimentology, micropaleontology, inorganic and organic geochemistry, radiocarbon and OSL dating). Within the offshore Holocene stratigraphic record, at least two event layers of likely tsunami backwash origin were identified based on their significantly different properties compared to the background shelf sediments. The uppermost tsunami layer (at a depth of 16-25 cm in most cores) displays an erosional contact at the base with heterogeneous compositional changes; its bounding radiocarbon ages allow a correlation with the AD 1755 Lisbon tsunami. Organic-geochemical markers, such as n-alkanes, polycyclic aromatic hydrocarbons, steroids and fatty acids, show an increased input of terrestrial matter in this offshore AD 1755 event layer.</p><p>A surprising discovery was another distinct high-energy deposit, i.e. a potential predecessor to the AD 1755 Lisbon tsunami, at a core depth of about 122-155 cm, which was <sup>14</sup>C-dated to approx. 3700 yrs cal BP. Due to its erosional base and coarse-grained composition (well-sorted medium sand), as well as the increased terrestrial influence (displayed by biomarkers), it can be assumed that this deposit originates from the backwash of a paleo-tsunami.</p><p>This multi-proxy approach with sedimentological, micropaleontological, inorganic and organic-geochemical criteria, enabled us to (1) identify of backwash tsunami deposits; (2) establish a recurrence interval; and (3) estimate the hazard potential for the related coastal areas. Results of the M152 cruise demonstrate for the first time that the depositional basins on the Algarve shelf have the potential to reliably archive Holocene tsunami backwash deposits. The low-energy environment of the outer Algarve shelf sets prime conditions for the preservation of tsunami backwash deposits. Thus, these geoarchives offer the possibility to study the mechanisms and hydrodynamics of backwash currents, and to investigate tsunami strata that are not preserved elsewhere.</p>

Offshore 1755 CE Lisbon Tsunami Deposit in the Southern Portuguese Continental Shelf

<p>The importance of tsunami hazard assessment is only possible if a complete dataset of events is available, allowing the determination of the recurrence intervals of the tsunamis adapted to local and regional conditions. One possible way to know these intervals is to study the offshore sedimentary record, looking for sediment remobilised and transported by the incoming tsunami waves and generated backwash currents. Even if these deposits are not of easy access (and not so well studied), the tsunami depositional signature has potential to be better preserved than those located onshore.</p><p>A multidisciplinary approach was performed to detect the sedimentary imprints left by the 1755 CE Lisbon tsunami event in three cores located in southern Portuguese continental shelf at water depths between 57 and 91 m. Age models based on <sup>14</sup>C and <sup>210</sup>Pb<sub>xs</sub> data allowed a probable correspondence with the 1755 CE Lisbon tsunami.</p><p>The present study was based in high-resolution analyses using several methodologies such as sand composition, grain size, inorganic geochemistry and microtextural features on quartz grain surfaces. The results yielded evidences for a tsunamigenic origin although no remarkable terrigenous signal is present. Spatial depositional differences of tsunami sediments were detected in the study area by differences in grain size, sand composition and simulated horizontal surface velocities. Also, the heterogeneous and mixing character of the 1755 CE Lisbon offshore tsunami deposits indicate more complex sedimentary conditions compared to the background sedimentation.</p><p>This study shows that in fact the sediment layers corresponding to a tsunami event can be preserved in mid to outer continental shelf environments (other extreme events such as storms were excluded trough hydrodynamic calculations), but its identification and characterization can be done only with a good assemblage of different proxies.</p><p><em>This is a contribution of ASTARTE project (FP7-Grant agreement no: 603839) and CIMA project (UID/MAR/00350/2013).</em></p>

Recent Advances in the Identification of Onshore and Offshore Tsunami Deposits from the Eastern Mediterranean Coastline of Israel

<p>The first physical field evidence for any dated tsunami event on the coast of Israel was discovered twenty years ago.  Since then, three campaigns of offshore core collections were completed with the aim of testing the validity of that interpretation, further completing the catalogue of known tsunami events, providing constraining data for models, determining associations with potential source tsunami-generating mechanisms, and assessing risk for purposes of emergency planning and coastal management.  Those follow-up coring campaigns provided many additional examples of anomalous sedimentary deposits that agreed with tsunami-derived interpretations and failed to fit criteria of other potential causes (e.g. floods, storms); reinforcing the theory that multiple tsunami events impacted that coastline and building a more complete record.  The interpretation of these offshore deposits has been improved by ongoing contributions from modern sedimentological studies following the set of recent megatsunamis.  Specifically, tsunami sediment characterization from modern tsunami studies has greatly improved the ability to recognize cryptic, anomalous deposits with higher confidence.  In addition, a small set of new land-based evidence has been identified, some of which match written historical records, and many that corroborate the offshore sedimentary record. In this presentation, a summary of these finds and the latest, most updated catalogue of events based on physical sedimentary deposits will be presented highlighting knowledge gained regarding variations in the efficacy of various proxies in the tsunami ‘tool box’ with relationship with this particular stretch of coastline.</p>